Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
The development of deep-looking electromagnetic tools has a long history. Such tools are used to achieve a variety of different objectives. Deep looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high resistivity formations and the power available down-hole.
At the ultra-deep scale, technology may be employed based on transient field behavior. The transient electromagnetic field method is widely used in surface geophysics. Examples of transient technology are seen, for example, in Kaufman et al., 1983, “Frequency and transient soundings”, Elsevier Science.; Sidorov et al., 1969, “Geophysical surveys with near zone transient EM.” published by NVIGG, Saratov, Russia; and Rabinovich et al., 1981, “Formation of an immersed vertical magnetic dipole field”: J. Geologiya I Geofizika, N 3. Typically, voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation. Electric currents diffuse outwards from the transmitter into the surrounding formation. At different times, information arrives at the measurement sensor from different investigation depths. Particularly, at a sufficiently late time, the transient electromagnetic field is sensitive only to remote formation zones and does not depend on the resistivity distribution in the vicinity of the transmitter (see Kaufman et al., 1983). This transient field is especially important for logging. Use of a symmetric logging tool using transient field measurements for formation detection is discussed, for example, in U.S. Pat. No. 5,530,359, issued to Habashy et al.
U.S. Pat. No. 5,955,884, issued to Payton et al. discusses methods for measuring transient electromagnetic fields in rock formations. Electromagnetic energy is applied to the formation at selected frequencies and waveforms that maximize the radial depth of penetration of the magnetic and electric energy. Payton comprises at least one electromagnetic transmitter and at least one electric transmitter for applying electric energy. The transmitter may be either a single-axis or multi-axis electromagnetic and/or electric transmitter. In one embodiment the TEM transmitters and TEM receivers are separate modules that are spaced apart and interconnected by lengths of cable, with the TEM transmitter and TEM receiver modules being separated by an interval of from one meter up to 200 meters, as selected. Radial depth of investigation is related to the skin depth, δ=√{square root over (2/σμω)}, which in turn is related to frequency. Lower frequency signals can increase the skin depth. Similarly, the conductivity of the surrounding material inversely affects the skin depth. As conductivity increases, the depth of investigation decreases. Finite conductivity casing of the apparatus therefore can reduce the depth of investigation.
State-of-the-art measurement-while-drilling (MWD) technology introduces a new, deep (3-10 meters) scale for an electromagnetic logging application related to well navigation in thick reservoirs. One problem associated with the MWD environment is the presence of the metal (imperfectly conductive) drill pipe close to the area being measured. This pipe produces a very strong response and significantly reduces the sensitivity of the measured EM field to the effects of formation resistivities and remote boundaries. Previous solutions for this problem typically comprise creating a large spacing (up to 20 meters) between transmitter and receiver (as discussed in U.S. Pat. No. 6,188,222 B1, issued to Seydoux et al.). The sensitivity of such a tool to remote boundaries is low. Currently, Stolar Horizon, Inc. is developing drill string radar, DSR, for CBM (Coal Bed Methane) wells. DSR provides 3-D imaging within a close range of the wellbore.
Currently, induction tools operate to obtain measurements in the presence of a primary field. In measurement-while-drilling method, examples of such techniques are the Multiple Propagation Resistivity (MPR) device, and the High-Definition Induction Logging (HDIL) device for open hole. One or more transmitters disposed along a drill tool act as a primary source of induction and signals are received from the formation at receiver coils placed at an axial distance from the transmitters along the drill tool. A disadvantage of both MPR and HDIL methods is that the primary source of induction from the transmitter is always present during the time frame in which the receivers are obtaining measurements from the formation, thereby distorting the intended signal. This can be solved by using pulse excitations such as is done in a transient induction tool where signals are measured during time intervals between excitation pulses.
In a typical transient induction tool, current in the transmitter coil drops from its initial value I0 to 0 at the moment t=0. Subsequent measurements are taken while the rotating tool is moving along the borehole trajectory. The currents induced in the drilling pipe and in the formation (i.e. eddy currents) begin diffusing from the region close to the transmitter coil in all the directions surrounding the transmitter. These currents induce electromagnetic field components which can be measured by induction coils placed along the conductive pipe. Signal contributions due to the eddy currents in the pipe are considered to be parasitic since the signal due to these currents is much stronger than the signal from the formation. In order to receive a signal which is substantially unaffected by the eddy currents in the pipe, one can measure the signal at the very late stage, at a time in which the signals from the formation dominate parasitic signals due to the pipe. Although the formation signal dominates at the late stage, it is also very small, and reliable measurement can be difficult. In prior methods, increasing the distance between transmitter and receivers reduces the influence of the pipe and shifts dominant contribution of the formation to the earlier time range. Besides having limited resolution with respect to an oil/water boundary, such a system is very long (up to 10-15 m) which is not desirable and convenient for an MWD tool.
In conventional HDIL situations, a fixed “wound-counter-wound” turn ratio between two receiver electrodes R1 and R2 may be used. It is generally understood that such an arrangement advantageously achieves the objective of reporting a zero signal in the absence of anything but air proximate to the measurement tool. Typically, any residual signal that might be present due to particular implementation details may be calibrated out of the system by performing air-hang calibration.
It is also known to conduct logging-while-drilling (LWD) operations using multiple frequencies. Conventionally, a wound-counter-wound configuration preferably has a turn ratio (the number of loops in respective receiver coils) that is adjusted to achieve substantially zero signal during air-hang calibration. This calibration suffices for a single frequency, and any residual non-zero signals are characterized mathematically and subtracted out using digital processing techniques. All residual non-zero signals throughout a range of frequencies can be calibrated with an “air hang.”
There are several factors that make it difficult to perform both geo-steering and ahead of the bit induction measurements while drilling. One factor is the conductive drill string (metal pipe) which has a finite, non-zero conductivity. The measured transient induction signal is severely affected by the eddy currents in the pipe, limiting resolution of the measurements to the formation parameters, specifically, to a water-oil boundary in case of geo-steering. By increasing a distance between transmitter and receiver it is possible to reduce the influence of the drill while increasing contribution into the signal from the formation. Besides having a limited resolution with respect to the parameters of interest such system might be too long (up to 10-15 m) for MWD applications.
In case of the measurements ahead of the bit positioning of the induction system with respect to the drill bit represents a second complicating factor. Indeed, the first 3-4 meters in the vicinity of the drill bit are not allowed for the transmitter/receiver coils' placement since this space is occupied by the equipment supporting the drilling process. In case of the system looking about 5 m ahead of the bit, the transmitter/receiver system would be separated from the region of interest by 8-9 m distance. Unfortunately, none of the existing resistivity tools have such depth of investigation.
The above mentioned circumstances emphasize an importance of deep-looking and relatively short EM (electromagnetic) system applicable for both geo-steering and ahead of the bit measurements while drilling.
Efforts to alleviate known problems with the presence of a conductive drillpipe in regions where transient EM sensing operations are taking place have been proposed in the prior art.
U.S. published patent application no 2007/0216416, filed on behalf of Gregory B. Itskovich on Mar. 6, 2007 and entitled “Combination of Electromagnetic and Magneto-static Shields to Perform Measurements Ahead of the Drill Bit” (“the Itskovich '416 application”) discloses one approach for ahead-of-bit measurements. The Itskovich '416 application is commonly assigned to the assignee of the present invention and is hereby incorporated by reference herein in its entirety. Transient MWD resistivity measurement systems are further disclosed in U.S. Pat. Nos. 7,167,006; 7,150,316; 7,046,009 commonly assigned to the assignee of the present invention. The foregoing references describe different techniques allowing for performing transient electromagnetic measurements in the presence of conductive support (pipe). All the disclosed techniques require relatively long (several meters) segments of copper shielding and long (several meters) segments of ferrite shielding to be placed substantially between the transmitter and receiver coils. Indeed, experimental results suggest that such shielding approaches may not be entirely practical from a mechanical standpoint due to the requisite lengths of shielding.